1. Field of the Invention
The present invention relates to polyurethane joint sealing compounds which are useful for sealing, caulking and glazing operations, particularly in the construction industry. More specifically, the present invention is directed to a reliable, low-cost, one or two part, non-sag curable polymeric sealants which can provide a soft, low modulus seal suitable for joints which are subject to substantial variations in width and movement.
2. Discussion of the Prior Art
Polyurethane adhesives or sealants and similar-type curable polymeric materials, such as polyacrylic adhesives or sealants, in general have relatively high bond strengths, flexibility, shock and impact resistance, fatigue resistance, and the like. The adhesion or bonding of such compounds arises primarily from a polymerization reaction (otherwise known as curing) which is designed to occur once the adhesive or sealant is applied. As the adhesive or sealant cures, it chemically bonds to most contacting surfaces and transforms from a soft, workable liquid or semi-liquid to a firm, resilient, elastomeric solid. The curing process is generally not reversible, and once the polyurethane adhesive or sealant is applied and allowed to cure, the material generally cannot be removed and re-applied.
Not surprisingly therefore, polymeric adhesives or sealants are generally sold in a non-cured form, containing polymerizable components which are designed to cure upon application. In general, such adhesives or sealants are created from a prepolymer component and a curative component. Typically for polyurethanes, the prepolymer component is an organic polyisocyanate having excess isocyanate functional groups in a weight percent amount of about 2% to about 10%, more typically 4% to about 7%. The curative is generally a reactive hydrogen-containing material.
Two-step systems generally require mixing just prior to application, and such mixing is often found to be inconvenient and burdensome. Therefore one step systems are generally preferred by end users. One step systems have only a single component to apply. This component will generally cure due to ambient conditions, most often due to moisture in the air. In polyurethane systems, moisture in the air can react with the isocyanate reactive sites and cause polymerization directly or can react with an intermediary, such as oxazolidine or ketimine, to produce a reaction product (typically an amine) which in turn initiates curing.
Regardless of whether the system is one component or two component, the adhesive or sealant will generally also contain sufficient urethane catalyst to give the desired cure time, so the adhesive can be applied prior to curing. Such one and two part polyurethane adhesives or sealants are widely employed in many industries, such as the construction industry and motor vehicle industry.
For application to vertical areas, such as wall surfaces or window frames, a sealing material is required to be of the non-sag type, i.e. substantially free from sagging or slumping immediately after application. "Non-sag" type sealing material should flow easily by external forces during its mixing and application, but should not flow (and therefore should exhibit a substantial increase in apparent viscosity) when it is at a stand still. Such behavior is generally described as thixotropic.
Thixotropic polyurethane sealants and adhesives are known generally in the art, but are not without their problems. Such materials often require a precise formulation which may be unforgiving if any errors occur during the manufacturing process. Moreover, such materials are sometimes also sensitive to temperature, and unfortunately a large portion of the thixotropic sealant or adhesive market involves large temperature extremes (automobile, building and roadway surfaces can have rather extreme variations in temperature). Some such sealants or adhesives do not work well when the width of the space sealed by the sealant varies by 25% or more.
Many known thixotropic sealants are expensive and difficult to manufacture. Such sealants or adhesives may be thixotropic but may nevertheless be undesirable due to a high yield point. A high yield point occurs when sealant movement is difficult to initiate, such as when removing the sealant from its storage container; once in motion however the material becomes much more flowable.
Urethane base sealing materials known in the art generally comprise a urethane prepolymer, a plasticizer, adhesion improver, catalyst and pigment or the like, together with a filler. The filler contributes not only to increasing the weight and thickness of sealing material, but also to control the hardness of cured the material.
Conventional fillers and reinforcing agents for polyurethanes and plastics are calcium carbonate, carbon black, diatomaceous earth, titanium dioxide, silica, talc, calcium silicate, Wollastonite, Asbestine, Kaolin, barium sulfate, graphite, hydrated alumina, chrysotile, serpentine, peralite, vermiculite, mica, crocidolite, zirconium silicate, barium zirconate, calcium zirconate, magnesium zirconium silicate, glass beads, fiberglass, titanium dioxide, PMF mineral fiber, nylon fiber, polyester fiber, alpha-cellulose fiber, polypropylene fiber, and the like. However, conventional fillers which are satisfactory for a joint sealant generally do not provide the desired combination of low modulus and non-sag properties when used alone or in combination with other fillers. The mixing of different fillers has generally been found to provide no synergistic result.
Adhesives without fillers generally have low sag resistance, and therefore fillers have generally been incorporated to develop or increase sag resistance where such resistance is needed. Unfortunately, this technique also increases the viscosity of the component or components containing the filler, making it more viscous and more difficult to apply.
Hence, loading with high levels of filler increases sag resistance but typically results in difficulties in obtaining satisfactory mixing, because high pressure mixing (or shear) equipment and high pressure pumping equipment are generally needed. Furthermore, high filler loading in many cases tends to lower the strength of the adhesive bond and typically decreases elasticity.
If the filler cannot provide sufficient non-sag properties, a non-sag agent might also be used. Common non-sag agents include colloidal silica, PVC plasticizers, castor oil derivatives, bentinite clays. Although non-sag agents provide non-sag properties superior to most fillers, like fillers, non-sag agents also increase the apparent viscosity of the mixture, resulting in considerable reductions in mixing or other operational efficiencies.
As the amount of non-sag agent is increased, the physical properties of the sealing material generally deteriorates, because hardness and modulus are generally increased and elongation and adhesion strength are generally reduced. For this reason, the use of a great amount of an non-sag agent is generally not desirable. However, there have been known in the art few, if any, non-sag agents which are useful for urethane sealing material in only a small amount. Therefore conventionally known non-sag agents are often used in combination with one or more fillers.
Some sealants employ a non-sag elastomeric type silicone caulking composition which provides a soft low modulus elastomeric product. However such joint sealants are generally expensive, can cause staining, and often cannot be painted.
A two-component epoxy-type polyurethane joint system has been on the market for many years under the trade name DYMERIC and is disclosed generally in U.S. Pat. No. 3,445,436. This two-component sealant has non-sag properties and, when cured, has a low modulus comparable to many other sealants. However, it is a two part system which requires mixing just prior to application.
One-component non-sag, moisture curing polyurethane caulking compositions have been very popular for simple joint sealing operations due to the economy of application, but they present certain problems. It has generally not been possible to provide a low modulus one-component polyurethane sealant having the desired softness, sag resistance and flow characteristics which could be easily manufactured at a relatively low cost and which meets the Federal specifications for Class A, cold-applied elastomeric type joint sealants. Such Federal specifications have been in existence for more than a decade and were initiated by the National Bureau of Standards and set forth in Federal specifications TT-S-00227E and TT-S-00230C.
Many known polyurethane sealants have not been able to meet the Class A requirement, because they have excessive durometer hardness (e.g., a Shore A durometer hardness of about 60 or greater when compounded). Manufacturers of caulking compositions unable to obtain such Class A ratings are often severely limited in the marketing of their products, because contracts involving government construction projects generally require Class A sealants meeting the Federal specification.
Conventional compounding techniques make it possible to vary the elastomeric properties and flow properties of a polyurethane sealing composition but it has generally been very difficult to achieve the desired combination of extrudability, non-sag and low modulus merely by judicious choice of compounding ingredients, conventional non-sag agents and/or fillers.